</font><font color="blue" class="small">( After heating up the glow plugs for about 15 seconds it will barely turn over. )</font>
The glow plugs won't make it hard to turn over, just hard to start in cold weather if they're not working.
Before you spend the money, test the battery with a volt meter. You should have just above 12V if it's fully charged, and around 14-14.5 running. if the battery is okay, you may have a compression problem.
As for the throttle, the parts are not expensive, and I think you have the right idea to go with the factory part. If the hour meter is correct, you should have many many more hours of great use. Keep us up to date.
John